The objectives of this research program are an analysis of the pathogenesis of murine mammary cancer using several model systems recently developed. The emphasis of the current research is on the inhibition of mammary tumorigenesis by Selenium in both viral and chemical carcionogen-induced mammary tumors. Experiments will be perfomed to determine the stages of mammary tumorigenesis which are sensitive to inhibition by Selenium. The second area of research to be pursued will be the development of appropriate in vitro cell culture conditions to analyze the effects of inhibitory compounds (i.e., Selenium) on the growth of normal, preneoplastic and neoplastic mammary cells.